An infrared (IR) radiation detector responds to thermal energy such as, for example, infrared radiation radiated by objects such as, for example, animals, automobiles, airplanes, gunfire, missiles, explosions, fire, etc. This thermal energy is typically not visible to the human eye. Accordingly, by using an infrared radiation detector, objects that are not visible may be perceived and/or alternative views of visible objects may be obtained.
Materials are known that may be excited by the absorption of infrared energy to provide a useful electrical signal for an infrared radiation detector. Some infrared radiation detectors take advantage of the fact that a change in conductivity occurs in the material when heating with infrared radiation. This change in conductivity is used to modulate a signal impressed across the infrared sensitive material. These infrared radiation detectors are satisfactory for some purposes, but the reaction time in such infrared radiation detectors is inherently long since the operation depends entirely upon a thermal affect.
Another type of infrared radiation detector employs certain materials which operate on the photoconductive principal such that when the material is excited by absorption of infrared energy, there is a change in conductivity in the material. Certain of these photoconductive materials can be used and have very short time constants.
Lead salts provide some of the most sensitive materials for the detection of infrared energy at certain wavelengths and especially lead selenide and lead sulfide materials. An infrared radiation detector utilizing these materials usually comprises a thin film of lead selenide or lead sulfide on a substrate with electrical leads connected to opposite sides of the thin film or layer. The thin film of lead salt can be formed by either vacuum evaporation or deposition onto the substrate or by chemically depositing onto the substrate from a solution.